In the field of electrochemistry there is a well-known electrochemical cell known as a chlor-alkali cell. In this cell, an electric current is passed through a saturated brine (sodium chloride salt) solution to produce chlorine gas and caustic soda (sodium hydroxide). A large portion of the chlorine and caustic soda for the chemical and plastics industries is produced in chloralkali cells.
Such cells are divided by a separator into anode and cathode compartments. The separator characteristically can be a substantially hydraulically impermeable membrane, e.g., a hydraulically impermeable cation exchange membrane, such as the commercially available NAFION manufactured by the E. I. du Pont de Nemours & Company. Alternatively, the separator can be a porous diaphragm, e.g., asbestos, which can be in the form of vacuum deposited fibers or asbestos paper sheet as are well known in the art. The anode can be a valve metal, e.g., titanium, provided with a precious metal coating to yield what is known in the art as a dimensionally stable anode.
One of the unwanted byproducts present in a chlor-alkali cell is hydrogen which forms at the cell cathode. This hydrogen increases the power requirement for the overall electrochemical process, and eliminating its formation is one of the desired results in chlor-alkali cell operation.
It has been estimated that 25 percent of the electrical energy required to operate a chlor-alkali cell is utilized due to the formation of hydrogen at the cathode. Hence, the prevention of hydrogen formation, e.g., by reacting hydrogen with oxygen at the cathode resulting in the formation of hydroxide, can lead to substantial savings in the cost of electricity required to operate the cell. In fairly recent attempts to achieve cost savings and energy savings in respect of operating chlor-alkali cells, attention has been directed to various forms of what are known as oxygen (air) cathodes. These cathodes prevent the formation of molecular hydrogen at the cathode and instead reduce oxygen to form hydroxyl ions. Savings in cost for electrical energy are thereby achieved.
One known form of oxygen (air) cathode involves use of an active layer containing porous active carbon particles whose activity in promoting the formation of hydroxide may or may not be catalyzed (enhanced) using precious metal catalysts, such as silver, platinum, etc. Unfortunately, however, the pores of such active carbon particles may become flooded with the caustic soda thereby significantly reducing their ability to catalyze the reduction of oxygen at the air cathode and resulting in decreased operating efficiency. In an attempt to overcome these difficulties in flooding of the active carbon, hydrophobic materials, e.g., polytetrafluoroethylene (PTFE), have been employed in particulate or fibrillated (greatly attenuated and elongated) form to impart hydrophobicity to the active carbon layer, per se, and/or to a protective (wetproofing) or backing sheet which can be laminated or otherwise attached to the active layer. Thus, PTFE has been employed in both active layers and in backing (wetproofing) layers secured thereto.
The present invention provides a process for starting up an oxygen (air) cathode, e.g., of the three-component variety, by what amounts to essentially zero pressure start-up using hot alkali, e.g., at 60.degree. to 85.degree. C. According to one preferred embodiment of this invention, low current density is used for the period of start-up, e.g., 30 to 90 milliamps/cm.sup.2 for a sufficient period of time to accomplish start-up, viz., 1 to 24 hours. According to another preferred embodiment of this invention, the start-up is performed using a 3 to 10 times stoichiometric amount of air or oxygen required to conduct the oxygen reduction reaction desired in the electrolytic cell. Preferably CO.sub.2 -free air is used. Such excess of air is customarily employed in the process of this invention at 0 to 15 inches of water pressure.
U.S. Pat. No. 3,403,083 to Currey et al is directed to imposing decomposition voltage cross electrodes and recirculating the sodium chloride anolyte using greater brine feed rate than the amount which flows through the diaphragm. The temperature of the anolyte is controlled at between 75.degree. to 80.degree. C. versus the 65.degree. C. normally used. The pH of the anolyte is controlled by the addition of acid to the system.
DuBellay et al, U.S. Pat. No. 3,527,690, is directed to a means for controlling and maintaining gas pressure within an oxygen depolarized cathode.
Hora et al, U.S. Pat. No. 3,985,631, discloses a pretreatment and startup of Nafion cell membranes involving immersion of the membrane and caustic while imposing a potential of from 6.5 to 8.5 volts followed by immediately passing a direct current therethrough and then regulating the voltage to maintain the desired current density.
Welch, U.S. Pat. No. 4,135,995, discloses using a cathode made of an intercalation compound of carbon and fluorine and were in the oxidant, e.g., air, is bubbled through the catholyte liquor, viz., added directly to the catholyte.
LaBarre, U.S. Pat. No. 4,221,644, is directed to operation of an oxygen (air) cathode with the stated objective of minimizing voltage necessary for operation by controlling the pressure of the air feed side to obtain positive gauge pressure, e.g., of 110 g/cm.sup.2 (44 inches of water), for example, as stated in Example 1 with the stated range of pressure differential in the range of 0.25 through 500 g/cm.sup.2 (0.1 to 200 inches of water); controlling the total flow of the air feed side; humidifying the air on the air feed side and elimination of CO.sub.2 from the air feed.
At column 4, lines 9 to 14, LaBarre states that the air is fed to the interior of the oxygen compartment at a positive gauge pressure so as to accomplish a total flow rate in excess of the theoretical stoichiometric amount of oxygen necessary for the reaction.
In the first full paragraph of column 10 of LaBarre, viz., lines 9 to 20, LaBarre states that a pressure differential is applied across the porous cathode 18 so that the pressure in the oxygen compartment 24 is higher than that in the cathode compartment 22. The patentee further states that the increased pressure, which may be zero gauge to bubble through but due to the electrolyte head may be negative absolute, assists in mass transfer of the oxidizing gas such as air with CO.sub.2 removed into the cathode 18 thereby preventing oxygen depletion in the reaction zone within the cathode 18 and leading to a longer cathode lifetime. The reference in LaBarre to the use of zero gauge to bubble through pressure is different from the start-up procedure of this invention in its use of the claimed range of reduced starting pressure, viz., from 0 to 306 mm Hg (0 to 27 inches of water) in at least the following respects: (1) The air flow rate in the LaBarre patent is set according to the desired excess air based on the final operating current density, e.g., 2.5 times the stoichiometric requirement of air at 310 mA/CM.sup.2. In the procedure of this invention, it was found to be beneficial to cathode performance to start at an air flow rate in excess of that intended for normal operation and continue this "high" rate for the duration of the break-in period. Following the break-in, the air rate is decreased to preset levels. (2) Similarly, whereas this LaBarre patent sets the air pressure at that level planned for standard operation, the procedure of this invention prescribes that the pressure begin at zero and be increased to operating pressure only as break-in is concluded. In this manner, it is believed that the best surface area within the cathode can be wetted and maintained until operation is achieved, and the best voltage and life performance is yielded by the cathode so started.